Hi im no webmaster but i had the same problem a few weeks ago,
What i found was this.... it works in FF but not IE.... im yet to find out how this can be achived in IE.
whilst in dreamweaver (cs3) create your box as you would do,
Open the CSS from the top right panel,
select borders, and select the type of your border you'd like to use, thicknes, solid, thin and give it the color youd like too. clicking ok to close the css properties box.
Then look through properties of the css - scroll down a bit and locate the following - ignore the (4px) as i have used this as a guide only...
-moz-border-radius-bottomleft (4px)
-moz-border-radius-bottomright (4px)
-moz-border-radius-topleft (4px)
-moz-border-radius-topright (4px)
This may not be what ye looking for, but in FF it will give you your desired effect, Open IE, and they will be square....
If you find out how these corners can work in IE I would be very much appreciated if you could repost here...
Good luck....